A proposal for obtaining predictions of correlations between positions and momenta from the peaks of the Wigner function is studied. An implementation of this proposal based on approximate Wigner functions of a special form obtained from semiclassical wave functions is shown to be unreliable because the exact Wigner functions do not have this form. This approach has been used to argue that classical general relativity is predicted from the semiclassical wave function of the Universe, and this must now be reconsidered. A new measure of correlation, involving projection onto coherent states, is proposed for predicting classical correlations from a general Wigner function. In the harmonic oscillator, it predicts a correlation of position and momentum given by the Hamiltonian equated to the classical energy.
The quantum wave-function is at the heart of quantum mechanics. It is the physical state of a quantum system before a measurement is taken. This wave-function is described mathematically by the formulation of matrix mechanics and represents the evolution of the quantum wave-function. In contemporary philosophy of physics, the wave-function’s nature of space is hotly debated. Contemporaries such as David Albert, who posits an ontology which is based off the mathematical features of the wave-function, claim that the dimensionality of our physical space is proportional to 3N, where N is equal to the number of particles in the universe. Others such as Bradley Monton claim quantum mechanics to be a false theory from the outset due to its incompatibility with general relativity and posit a much more realist property-based-physics ontology. However, these ontologies have their problems. Albert’s ontology takes the mathematics of the quantum wave-function at face value and disregards the distance properties of metric space to misconstrue the phase space nature of the 3N-configuration space as a metric space. Monton, on the other hand, questionably grounds his thesis on the incompatibility of quantum mechanics with general relativity and on common sense rather than on observation and experimentation, as required by the scientific method. In this thesis, I discuss the viewpoints of Albert and Monton in detail and explore the nature of space in the quantum wave-function in an attempt to reach a better understanding of the fundamental nature of space in the quantum wave-function.
Despite its importance in general relativity, a quantum notion of general covariance has not yet been established in quantum gravity and cosmology, where, given the a priori absence of coordinates, it is necessary to replace classical frames with dynamical quantum reference systems. As such, quantum general covariance bears on the ability to consistently switch between the descriptions of the same physics relative to arbitrary choices of quantum reference system. Recently, a systematic approach for such switches has been developed. It links the descriptions relative to different choices of quantum reference system, identified as the correspondingly reduced quantum theories, via the reference-system-neutral Dirac quantization, in analogy to coordinate changes on a manifold. In this work, we apply this method to a simple cosmological model to demonstrate how to consistently switch between different internal time choices in quantum cosmology. We substantiate the argument that the conjunction of Dirac and reduced quantized versions of the theory defines a complete relational quantum theory that not only admits a quantum general covariance, but, we argue, also suggests a new perspective on the ‘wave function of the universe’. It assumes the role of a perspective-neutral global state, without immediate physical interpretation that, however, encodes all the descriptions of the universe relative to all possible choices of reference system at once and constitutes the crucial link between these internal perspectives. While, for simplicity, we use the Wheeler-DeWitt formulation, the method and arguments might be also adaptable to loop quantum cosmology.